Currently, the best tool for feedback is DPS meters. But while DPS meters work, they are a very blunt instrument. They don't account for differences in gear, or fights, or even tell what number you should be aiming for.
I think a better DPS meter could be made, but it would probably require the game developers to implement.
Currently, DPS meters compare you to the other players in the current fight. It would be better if the DPS meter compared you to the overall historical performance of people with your item level.
Let's start by recording everyone's performance on individual boss fights. Note the boss, DPS done, and the item level of the player. Once you aggregate all the records, you can tell for a given boss and item level, what the top DPS was, or what the median DPS was.
That gives you a target number. If the top DPS on Boss A at i500 is 10k, you can tell the player after Boss A: "You did 6k damage. The top DPS was 10k." That is concrete feedback. The player can't blame her gear or the fight mechanics.
Of course, using the top DPS mark is probably bad, because it would be a very lucky parse and probably individuals doing something excessive to hit that mark. A better target number would be something like one standard deviation above the median. Or possibly target the range between the median and one standard deviation.
The advantage of using this mechanism, which looks back at the history of all the people doing the fight is that it nullifies variables and fight mechanics. Because the amount of data collected is large, a few lucky parses or exceptional players do not skew the results. It provides a viable target number that people know for a fact is within the capabilities of the class and gear.
As well, this doesn't necessarily involve the entire raid. You aren't being compared to other people you know, but to the entirety of the WoW community.
If feedback is vague, you can always make excuses as to why you don't need to improve. For the DPS to improve, they first need unequivocal proof that improvement is necessary. This Historical DPS Meter would provide that feedback.